This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Blood-brain barrier disruption is a hallmark of nervous system diseases associated with vascular rupture, such as stroke, multiple sclerosis (MS), brain glioblastomas and spinal cord injury. However, the molecular and cellular mechanism of the contribution of blood components to CNS pathogenesis remains poorly understood. Our previous studies identified that fibrinogen, a major blood factor deposited in the nervous system after BBB disruption, inhibits peripheral nerve regeneration and exacerbates inflammatory demyelination in the central nervous system in an animal model for MS. The specific hypothesis in this proposal is that BBB disruption that leads to leakage of fibrinogen in the CNS is responsible for microglial activation. Our hypothesis is based on the observations that: 1. Using two-photon microscopy, microglia respond very rapidly by process extension and isolation of the traumatized sites to blood vessel damage in both the brain and spinal cord;2. Fibrinogen activates microglia in vitro via signaling through the CD11b/CD18 integrin receptor resulting in a dynamic rearrangement of the actin cytoskeleton resulting to increase in phagocytosis;3. Fibrinogen depletion in vivo results in decreased microglial activation in an animal model for MS. Based on these observations, the experimental focus of this proposal is on the direct demonstration of microglial activation by BBB disruption and fibrinogen leakage using in vivo imaging in the mouse brain and spinal cord. Since BBB disruption and microglial activation are hallmarks for several neurodegenerative diseases, we expect our work to provide a state-of-the-art demonstration of the molecular -crosstalk between blood factors and the brain parenchyma as it relates to glial cell activation and the development of CNS pathology.